1. Field of the Invention
The invention relates to a control device for a vehicle and, more particularly, to a control device for a vehicle that includes an engine and an electric motor as driving force sources and in which the electric motor is coupled to a drive wheel via a transmission.
2. Description of Related Art
In recent years, in terms of environmental protection, it is desired to reduce the amount of emission of exhaust gas from an engine (internal combustion engine) mounted on a vehicle and to improve a fuel consumption rate (fuel economy). As a vehicle that satisfies these requirements, a hybrid vehicle equipped with a hybrid system that includes an engine and an electric motor as driving force sources has been practically used.
There is, for example, known one hybrid system mounted on a vehicle (for example, see Japanese Patent Application Publication No. 2010-120639 (JP 2010-120639 A)). The system includes a power split mechanism, a second motor generator, a transmission and an electrical storage device (battery). The power split mechanism includes a sun gear, a ring gear and a planetary carrier (pinions) as rotating elements. The power split mechanism distributes the output of an engine to a first motor generator and a transmission shaft (ring gear shaft) (or combines the output of the engine and the output of the first motor generator and then outputs the combined output to the transmission shaft). The transmission is provided between the second motor generator and drive wheels (output shaft). The electrical storage device is able to exchange electric power with the first and second motor generators. Power from the second motor generator is output to the drive wheels via the transmission.
In addition, some transmissions applied to hybrid vehicles set a gear (speed ratio) with the use of clutches and/or brakes, which are frictional engagement elements, and a planetary gear unit. For example, there is an automatic transmission that includes two hydraulic brakes as frictional engagement elements and that shifts between a gear at which one of the brakes is engaged and the other one of the brakes is released (for example, low-speed gear) and a gear at which the other one of the brakes is engaged and the one of the brakes is released (for example, high-speed gear). Then, a hydraulic pressure for engaging or releasing the frictional engagement elements (brakes) of such a transmission is supplied by a mechanical oil pump that is driven by the power of an engine or an electric oil pump that is driven by an electric motor.
Note that, in a hybrid vehicle, there is the following technique described in Japanese Patent Application Publication No. 2007-216833 (JP 2007-216833 A) as a technique for starting or stopping a hybrid system. In the technique described in JP 2007-216833 A, when a vehicle start/stop switch is operated to an engine stop position (Off position or Ready-Off position) while the vehicle is travelling at a high speed, an engine stop is prohibited. By so doing, the amount of discharge of a battery does not exceed an upper limit discharge amount. Thus, the battery is protected.
Incidentally, in the above-described hybrid vehicle, when a user (driver, or the like) operates to stop the hybrid system (performs IG-Off operation) while travelling, the engine may stop. When the engine stops, the oil pump (mechanical oil pump, electric oil pump, or the like) also stops, a hydraulic pressure applied to the transmission decreases, causing the engaged frictional engagement element to be released. After that (after IG-Off while travelling), when restart operation (IG-On operation) is performed, the engine is restarted. When the engine is restarted, the hydraulic pressure applied to the transmission (hydraulic control circuit) rises accordingly, causing the frictional engagement element of the transmission to be engaged. At this time, the output shaft of the transmission is driven for rotation by the drive wheels, so there is a difference between the rotation speed of the output shaft and the rotation speed of an input shaft (motor generator side) of the transmission. When the transmission is engaged in such a state where there is a difference in rotation speed, a shock may occur at the time of the engagement. In addition, there is a concern about a drag of an engagement portion (frictional engagement element) of the transmission.
As measures against the above inconveniences, at the time of starting the engine, it is conceivable to execute control (synchronization control) for driving the second motor generator to cause the input-side rotation speed of the transmission to match with the output-side rotation speed of the transmission. In this case, when the second motor generator cannot be driven (torque cannot be output) due to an output limit (Wout) of the battery at low temperatures, or the like, it is not possible to reduce an engagement shock at the time of a restart or a drag of the engagement portion.
Note that the technique described in JP 2007-216833 A is a technique for, when the vehicle start/stop switch is operated to the engine stop position while the vehicle is travelling at a high speed, an engine stop is forcibly prohibited and does not take into consideration the case where IG-On operation is performed after an engine stop and, in addition, a decrease in hydraulic pressure in the transmission due to an engine stop while travelling is also not taken into consideration.